The invention generally relates to the startup of an electrochemical cell system.
A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM) that permits only protons to pass between an anode and a cathode of the fuel cell. Typically PEM fuel cells employ sulfonic-acid-based ionomers, such as Nafion, and operate in the 50° Celsius (C.) to 75° C. temperature range. Another type employs a phosphoric-acid-based polybenziamidazole, PBI, membrane that operates in the 150° to 200° temperature range. At the anode, diatomic hydrogen (a fuel) is oxidized to produce protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the protons to form water. The anodic and cathodic reactions are described by the following equations:Anode: H2→2H++2e−  Equation 1Cathode: O2+4H++4e−→2H2O  Equation 2
The PEM fuel cell is only one type of fuel cell. Other types of fuel cells include direct methanol, alkaline, phosphoric acid, molten carbonate and solid oxide fuel cells.
A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.
The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Electrically conductive gas diffusion layers (GDLs) may be located on each side of a catalyzed PEM to form the anode and cathodes of each fuel cell. In this manner, reactant gases from both the anode and cathode flow-fields may diffuse through the GDLs to reach the catalyst layers.
In general, a fuel cell is an electrochemical cell that operates in a forward mode to produce power. In the forward mode, the voltage across the stack has a first polarity. However, the electrochemical cell may be operated in a reverse mode in which the polarity of the voltage across the stack is reversed and the cell produces hydrogen and oxygen from electricity and water. More specifically, an electrolyzer splits water into hydrogen and oxygen with the following reactions occurring at the anode and cathode, respectively:Anode: 2H2O→O2+4H++4e−  Equation 3Cathode: 4H++4e−→2H2  Equation 4
An electrochemical cell may also be operated as an electrochemical pump in the reverse mode. For example, the electrochemical cell may be operated as a hydrogen pump, a device that produces a relatively pure hydrogen flow at a cathode exhaust of the cell relative to an incoming reformate flow that is received at an anode inlet of the cell. In general, when operated as an electrochemical pump, the cell has the same overall topology of the fuel cell. In this regard, similar to a fuel cell an electrochemical cell that operates as a hydrogen pump may contain a PEM, gas diffusion layers (GDLs) and flow plates that establish plenum passageways and flow fields for communicating reactants to the cell. However, unlike the arrangement for the fuel cell, the electrochemical pump cell receives an applied voltage such that the voltage across the stack has a polarity that is reverse of the voltage when operating as a fuel cell, and in response to the received current, hydrogen migrates from the anode chamber of the cell to the cathode chamber of the cell to produce hydrogen gas in the cathode chamber. A hydrogen pump may contain several such cells that are arranged in a stack.
Care must be exercised in shutting down and starting up an electrochemical cell stack to prevent conditions such as thermal combustion, stack membrane damage, and corrosion/oxidation of the stack components, such as carbon corrosion of the anode and cathode electrodes.